The present invention relates to a retractor (a winding device) for a seat belt and, in particular, to an improved retractor for a seat belt including a winding force reducing mechanism which reduces a webbing winding force when a seat belt is in use.
Conventionally, a seat belt device for holding an occupant such as a driver and a passenger in a vehicle safely in the seat comprises a webbing, a buckle, a mounting tool, a retractor and the like.
The retractor is a winding device which, when a seat belt is not in use, automatically draws a webbing and, when the seat belt is in use, can change the length of the webbing freely. In such a retractor, in general, the webbing is drawn out against the winding force of a winding spring thereby tightening the winding spring, while the webbing is drawn in by means of the winding force of the winding spring.
However, since the winding spring is tightened by of the draw-out the webbing, the winding force is normally increased as the webbing is drawn out. Therefore, in a seat belt device of a type that a webbing is drawn out from an ordinary winding device and is put on an occupant, when the occupant wears the seat belt, the webbing winding force becomes strong so that the force, due to the tension of the webbing, on the occupant is increased, which makes the occupant feel disagreeable. In particular, in a seat belt device of a type that a lap belt and a shoulder belt are used as a continuous webbing and a retractor is provided on the shoulder belt side, the webbing winding amount of the retractor increases. For this reason, if the webbing winding force of a winding spring is set such that it can provide a suitable value when restraining the occupant, then the webbing winding force becomes insufficient when the seat belt is stored, whereas, if the webbing winding force is set such that it can provide a sufficient value when the seat belt is stored, then the occupant restraining force of the webbing becomes too strong when the occupant wears the seat belt.
Thus, to solve the above problems, for example, as shown in Japanese Patent Publication No. Sho. 55-14661, there is proposed a seat belt winding device which comprises first and second winding springs respectively interposed between a base and a winding shaft for applying winding spring forces to the winding shaft in accordance with the draw-out of a seat belt, and control means for switching the connecting conditions of the first and second winding springs to select the winding spring force to be applied to the winding shaft to thereby apply a small winding spring force to the winding shaft when the occupant wears the seat belt.
That is, the seat belt winding device includes a winding force reducing mechanism in which the first and second winding springs are connected in series with each other, with the second winding spring having a stronger winding force than the first winding spring, and the control means switches the connecting conditions of the first and second winding springs with respect to the main body of the device and seat belt in such a manner that, during the storage of the seat belt, the control means applies the strong spring force of the second winding spring to the seat belt, while when the occupant wears the seat belt, it applies only the weak force of the first winding spring to the seat belt, thereby reducing the seat belt tensile force so as to reduce the sense of pressure applied to the occupant.
Also, according to a seat belt winding device which is disclosed in Japanese Patent Publication No. Sho. 55-39349, first and second winding springs are provided in such a manner that they are meshable and operable with each other through gears, a ratchet gear is provided on the second winding spring, a hook is provided on the ratchet gear which is energized such that it allows the rotation of a webbing in the draw-out direction thereof but prevents the rotation of the webbing in the winding or draw-in direction thereof, and the hook is engaged with the ratchet gear by a clutch mechanism in the webbing using portion, whereby, when the webbing is rotated in the draw-in direction thereof, the tightening of the second winding spring is increased through the gears by the rewinding force of the first winding spring to thereby act so as to cancel the rewinding force of the first winding spring, so that the sense of pressure to be given to the occupant can be reduced.
Further, as disclosed in Japanese Patent Unexamined Publication No. Hei. 1-141147, there is further proposed a seat belt winding device which includes a winding shaft for taking up a seat belt and winding springs for energizing the winding shaft in a webbing winding direction. In particular, the winding springs include a main spring for energizing the winding shaft constantly in the winding direction and an additional spring for applying its energizing force in the winding direction to the winding shaft in such a manner that the energizing force can be interrupted by interrupting means (clutch means), while the main and additional springs are arranged in parallel to each other.
In other words, the last-mentioned seat belt winding device includes a winding force reducing mechanism which, in the normal webbing winding operation, holds a sufficient winding force by means of the combined energizing force of the main and additional springs, and, when the occupant wears the seat belt, fits the seat belt to the body of the occupant only by means of the energizing force of the main spring, thereby being able to reduce the webbing winding force while the seat belt is placed on the occupant, so that an unnecessary sense of pressure is not given to the occupant.
However, in the seat belt winding device disclosed in the above-mentioned Japanese Patent Publication No. Sho. 55-39349, the clutch mechanism is operated only when the winding shaft is rotated in the webbing winding direction while the seat belt is placed on the occupant, and the second winding spring is tightened due to the rewinding force of the first winding spring and acts in such a manner that it cancels the rewinding force of the first winding spring. For this reason, when the seat belt is stored, the webbing winding force must be given only by means of the rewinding force of the first winding spring. This makes it difficult to reduce the size of the first winding spring and to miniaturize the retractor.
Also, in the seat belt winding device disclosed in the above-mentioned Japanese Patent Unexamined Publication No. Hei. 1-141147, the webbing winding force during seat belt storage is obtained by means of the combined energizing force of the two parallel arranged winding springs in the webbing winding direction and, when the occupant wears the seat belt, by interrupting the energizing force of the additional spring by use of the interrupting means, only the energizing force of the main spring can act as the webbing winding force. In this structure, if the energizing force of the main spring is set smaller than the energizing force of the additional spring in order to reduce the sense of pressure to be given to the occupant when the occupant wears the seat belt, then the energizing force of the additional spring becomes relatively large, which results in poor durability of the structure.
Further, in the seat belt winding device disclosed in the above-mentioned Japanese Patent Publication No. Sho. 55-14661 in which the first and second winding springs are connected in series with each other, since the winding torque of the first winding spring is smaller than the winding torque of the second winding spring, when the winding force reducing mechanism is removed by switching over the control means, then the first winding spring is suddenly tightened by the winding torque of the second winding spring and is thus quickly tightened to the full extent and, at the same time, when the seat belt is stored, the first winding spring is always tightened to the full extent. For this reason, if the winding force reducing mechanism is removed, then an excessive stress is applied to the first winding spring having a smaller winding torque, resulting in the poor durability of the first winding spring. That is, in order to enhance the durability thereof, the structure including the winding springs must be designed very carefully.